Treatment of zinc rich steel mill dusts for reuse in steel making processes

ABSTRACT

Crush-resistant, metallized agglomerates, containing substantially no zinc or lead and capable of being charged to various steel-making furnaces, are produced from dust recovered from the fumes of a basic oxygen or open hearth steel-making furnace by forming a moistened mixture of (a) the dust (b) finely divided solid carbonaceous material containing sufficient carbon to make the total equivalent fixed carbon content of the mixture about 5 to about 25 weight percent, based on the dry weight of the mixture, (e) a bonding agent in the range of about 5 to about 25 weight percent, based on the dry weight of the mixture, and (d) one or more additional strengthening agents in minor amounts; forming the resultant mixture into green agglomerates; hydrothermally hardening partially dried or dried agglomerates at a temperature substantially below the combustion or decomposition temperature of the carbonaceous material; and then heating the hardened agglomerates at an elevated temperature for a sufficient time to reduce the iron oxide to metallic iron and to reduce and volatilize the zinc, lead and other reducible impurities therein.

- United States Patent 1 Goksel TREATMENT OF ZINC RICH STEEL MILL DUSTSFOR REUSE IN STEEL MAKING PROCESSES [75] Inventor: Mehmet Adnan Goksel,Houghton,

- Mich. [73] Assignee: Board of Control of Michigan TechnologicalUniversity, l-loughton, Mich.

[22] Filed: Apr. 17, 1972 [2]] Appl. No.: 52 us. Cl. .L 75/25, 75/1,75/3 [51] Int. Cl C2lb 3/04 [58] Field of Search 75/3, 4, 5, 25, l

[56] References Cited UNITED STATES PATENTS 3,313,6l7 4/1967 Ban 75/53,524,743 8/1970 Hellwig 75/25 3,653,874 4/1972 Schulte 75/3 3,567,4283/1971 Svensson.... 75/3 2,4l7,493 3/1947 Holz 75/25 Primary ExaminerL.Dewayne Rutledge Assistant ExqminerleterD. Rosenberg Attorney-John W.Michaelet al.

5 X14445 T 64555 OXYGEN Nov. 6,1973

[ 5 7] ABSTRACT Crush-resistant, metallized agglomerates, containingsubstantially no zinc or lead and capable of being charged to varioussteel-making furnaces, are produced from dust recovered from the fumesof a basic oxygen or open hearth steel-making furnace by forming amoistened mixture of (a) the dust (b) finely divided solid carbonaceousmaterial containing sufficient carbon to make the total equivalent fixedcarbon content of the mixture about 5 to about 25 weight percent,

' based on the dry weight of the mixture, (e) a bonding 11 Claims, 1Drawing Figure C4? 011/4 C 6' 0(45 1% L cEmP #5741. \fnefxvr rf/zn m a50F "1 I Fae/mas j /0 26 l 24 27v I l A? cows/wee /52 I 02/52 I (HP/70M)[Mr/0M2) "i [Qua/25c a0 l r U/VDf/QS/ZED l P541515 i I J/ZI/V Al/TOCM/E6 F e/mas V JGQEEA/ & 20 r i 2 Nam FUR/V466 PAIENIEDNUV s 1915 kENb m ggBACKGROUND OF THE INVENTION This invention relates to a method forproducing hardened agglomerates from zinc-rich steel mill fume dustswhich are suitable for a steel making furnace charge. More particularly,this invention relates to the process for producing such agglomerateswhich are substantially free from zinc and lead impurities.

Primarily because of economic considerations, there has been a recenttrend towards an increased use of the basic oxygen processes for theproduction of steel. In this process, oxygen is blown onto or beneaththe surface of the molten metal. The fumes from such processes containconsiderable amounts of finely divided oxidized iron material, commonlyknown as BOF dust. The dust is collected by mechanical or electrostaticprecipitators to minimize atmospheric pollution and to recover thevaluable materials from the fumes. A certain amount of scrap metal iscustomarily charged to the process. Since some of the scrap typicallycontains zinc and/or lead the dust removed from the fumes usuallyinclude some zinc and lead, as well as iron oxide, because of thevolatility of zinc and lead, The dust recovered from the fumes from openhearth processes also can include zinc and/or lead.

The recovered BOF dust generally cannot be directly recycled to thesteel making furnace since the material is so fine that it will be blownout of the furnace during injection of the oxygen. Also, recycle of theBOF dust to the blast furnace is generally not possible because of thedeleterious effect of the contained zinc and lead on I the blast furnaceoperation. Therefore, it is desirable to agglomerate the BOP dust priorto recycle to the BOF furnace, and furthermore, it is desirable toprepare these agglomerates in such a way that their zinc and leadcontent might be removed conveniently while recovering the iron valuesin the steel making furnace.

A considerable amount of effort has been devoted to developing variousmethods for handling the dust and reclaiming the iron, zinc and leadvalues. Various leaching and pyroprocessing techniques have beenproposed for removing zinc and lead from the dust. The leachingprocesses employ various media including hydrochloric acid,sulfuric'acid, and ammonium carbonate. Generally, the leaching processesdo not completely remove zinc from the silicates and ferrites commonlypresent in BOF dust. 'The proposed pyroprocessing techniques generallyprovide a more effective removal of zinc and lead; however, they allhave certain practical disadvantages.

In one proposed prior art technique, such as described in U.S. Pat. No.3,386,816, a premoistened mixture of BOF dust and an acid and/or abinder, such as bentonite, is formed into balls. The green balls aredried and then introduced into a rotary kiln where they are tumbled inthe presence of a particulated, solid reductant, such as coke. The ballsare retained in the kiln for 45 minutes to 3 hours at a temperature ofl,600 to 2,500F to reduce the iron oxide and reduce and volatilize zinc,lead and other reducible impurities. With this process an extended timeperiod (with a corresponding cost for maintaining the high temperatures)is required to complete the desired reduction. Also, a separation stepis required to separate the unused solid reductant from the hardenedballs. This patent specifically teaches that the balls should containsmall amounts of internal solid reductant to prevent the compressivestrength of the dried balls from being reduced to an unacceptable levelfor handling and tumbling.

It has also been proposed to reduce pelletized steel mill dust byheating the green pellets in the presence of a reducing gas, such asreformed methane. in addition to requiring the use of natural gas whichis short in supply in this country, the hardened pellets produced bythis process often have low compressive strengths and cannot withstandthe extensive handling and transferring required by modern ore treatingprocesses.

in still another proposed prior art technique, such as disclosed by U.S.Pat. No. 3,262,771 the steel mill dust is mixed with about 25-35% coaland 0-1 0% limestone, the resultant admixture is pelletized, the greenpellets are sintered in a traveling grate furnace at a temperature ofabout l,800-2,300F to char bond the pellets, while calcining thelimestone and partially reducing the iron oxide in the pellets, and thepellets are finally heated in an electric furnace to convert the ironoxide into iron and volatilize the zinc, which is withdrawn as anoverhead vapor. U.S. Pat. No. 3,146,088 discloses a similar processwhere green briquettes having a porosity of more than 20% are formedfrom a mixture of blast furnace dust and open hearth furnace dust, whichhas been adjusted to have a predetermined carbon content. The porousgreen briquettes are then fired at l,0O0 to l,250C and the zinc and leadare recovered from the overhead vapor.

With these processes the unhardened green pellets must be carefullyhandled prior to introduction into the sintering furnace to prevent themfrom being broken up. Also, a considerable amount of the coal iscombusted during the sintering step so the sintering conditions must becarefuly controlled to insure sufficient carbon is available in thepellet to provide the desired reduction during the final heating step.Furthermore, the high temperatures required for the sintering stepresults in high operational costs because of the amount of heat energyrequired.

SUMMARY OF THE INVENTION A primary object of this invention is toprovide a process for producing high strength agglomerates fromzinc-rich steel mill waste dusts which are substantially free from zincand lead impurities and are suitable as a charge to a steel makingprocess.

Another object of this invention is to provide such a process whereby asolid reductant is incorporated within the agglomerate, so that thecontent thereof can be closely controlled to produce a rapidandsubstantially complete reduction of the iron, zinc, lead and otherreducible impurities, and yet the resultant .agglomerate has sufficientstrength to withstand handling and transportation prior ,to beingreduced.

The process of this invention broadly includes the steps of preparing amoistened admixture of a zinc-rich steel mill dust, finely divided solidcarbonaceous material and a bonding agent, aging the moistened admixturefor a sufficient time to hydrate the overburnt lime and/or slagcontained in the dust, forming this aged admixture into agglomeratedform, hydrothermally curing the agglomerates at a relatively lowtemperature to a hardened, crush-resistant form, and then heating thehardened agglomerates at an elevated temperature to completelyvolatilize the zinc and lead and metallize BRIEF DESCRIPTION oE "rffHEDRAWINGS The single drawing is a schematic flow diagram of the processof this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing,finely divided, particulate dust is removed from the fumes emanatingfrom a BOF furnace by a conventional fume abatement device 12, such asan electrostatic precipitator. Th dust typically comprises 40 to 70%iron, 3 to CaO, l to 5% SiO,, 0.01 to 15% Zn, 0.01 to 3% Pb and smallamounts of other impurities, such as MgO, A1 0 and the like. Because ofthe high temperature of formation, the BOP dust particles are relativelyfine .and are primarily spherically shaped. Typically, they have a sizedistribution of about 80% being less than 0.8 micron and about beingbetween about 0.1- and about 0.3 micron. Some of the BOP dust particlesmay be as fine as 0.05 micron. The separated dust is conveyed to aconventionalmixer 14, such as a pug mill, whrein it is mixed with afinely divided solid carbonaceous material, a bonding agent andsufficient water to form a moistened admixture capable of beingformedinto discrete agglomerated masses. Optionally, one or more strengtheningadditives and other steel mill waste fines, such as open hearth dust,scarfer grit, mill scale and spark box fines can be included in theadmixture.

Open hearth dust is quite similar to BOF dust but generally containsless slag and overburnt lime. Scarfer grit is produced when the surfaceof steel ingots are ground to remove the outer oxidized coating prior tovarious finishing operations. Various abrasive materials, such as sand,aluminum oxide and cast iron fines are mixed in this dust, so thecomposition thereof can vary. Mill scale consists primarily of metalliciron containing some iron oxides and contamination from hot toprefractories. The larger particles can be separated and returned to theblast furnace but the finer particles require agglomeration before theycan be reused and can be added to the admixture along with BOF dust. Thechemical composition of spark box fines is very similar to BOF dust, butthe particles are generally somewhat larger and consist mostly ofirregularly shaped, fused slag and iron oxides. It has been found thatamounts of one or more of these other steel mill waste fines, in amountsranging up to 75 weight of the total weight of the fines used in theadmixture, produces resultant, hardened agglomerates having superiorcompressive strengths. Scarfer grit is the preferred of these othersteel mill waste fines.

The moistened admixture is transferred to an aging apparatus 16 tohydrate the overburnt lime and/or slag typically contained in BOF dustand other steel mill waste fines used. The aged admixture is thentransferred to a conventional forming means, such as a pelletizer 18,wherein it is' formed into a discrete agglomerated form. The greenagglomerates are screened to a predetermined, substantially uniformsize, which is advantageous for charging to steel making furnace, in aconventional screening device 20. The under sized pellets are recycledto the pelletizer for further agglomeration. The sized, greenagglomerates are either introduced directly into a reaction chamber,such as an autoclave 22, or first transferred to a conventional drier24, where they are at least partially dried to remove a majority of themoisture content therein before being introduced into the autoclave. Inthe autoclave the green or dried agglomerates are heated to an elevatedtemperature under pressure and in the presence of moisture to effect ahardening and bonding of the particles into an integral, high strengthagglomerate. The aged admixture is preferably pel-letized and curedhydrothermally generally in' accordance with the process disclosed inUS. Pat. No. 3,235,371.

Although the hardened agglomerates removed from the autoclave havereasonably high compressive strengths after being cooled, their strengthcan be increased further by transferring them to a conventional drier 26where they are rapidly dried. In any event the hardened agglomerates canbe stored, transported and even charged to a steel making process if thezinc and lead content therein does not exceed a predetermined level atwhich furnace damage and/or undesirable metallurgical properties result.To remove the zinc and lead, the hardened agglomerates are introducedinto a conventional zinc removal furnace, such as a shaft furnace 28,where they are heated to an elevated temperature at which the iron oxidetherein is converted to metallic iron by the reducing action of thecarbonaceous material and the zinc, lead and the reducible impuritiesare reduced and volatilized. The zinc and lead are withdrawn as vaporfrom the shaft furnace and recovered from the overhead vapors in aconventional manner, such as in a condenser 30. The resultant metallizedagglomerates, from which substantially all of the zinc and lead has beenremoved, can be recycled to the BOF furnace, charged directly to a blastfurnace, or even charged to an electric steel making furnace whichordinarily cannot tolerate the presence of any appreciable amounts ofzinc.

The solid carbonaceous material used is a carbon containing material,such as coke, coke breeze, anthracite coal, bituminous coal, lignite,and the like. The carbonaceousmaterial is preferably pulverized to ap-'proximately the same general fineness aslthe mill dust; however, it canalso somewhat be coarser if desired.

The carbonaceous material should contain free carbon and supplysufficient carbon, in conjunction with the free carbon present in theBOF dust (and the other steel mill waste fines if used) to convert allthe iron oxide to metallic iron and reduce the zinc and lead oxidesduring the reduction step. The temperature of the hydrothermal hardeningstep is maintained substantially below the combustion temperature of thecarbonaceous material, so that substantially all the free carbon of theagglomerate remains available for reduction. Therefore, it is a simplemanner to determine the amount of carbonaceous material required toobtain the desired reduction, i.e., an amount corresponding to theavailable carbon stoichiometrically required to reduce the iron, zincand lad oxides in the agglomerate.

Preferably, the amount of carbonaceous material used is slightly inexcess of that stoichiometrically required; however, this excess shouldbe kept to a minimum to prevent unnecessary reductions in compressivestrength of the agglomerate and dilution of the resultant iron contentof the agglomerate. Generally, the amount of carbonaceous material usedin the mixture should contain sufficient carbon to make the totalequivalent fixed carbon of the mixture in the range of about 5 to about25, preferably about to weight percent, based on the total dry weight ofthe solids in the admixture.

The bonding agent used can be any material capable of bonding the dustparticles together under the hydrothermal conditions of th hardeningstep. Preferred bonding agents include the oxides, hydroxides andcarbonates of calcium and magnesium, and mixtures thereof, with lime andhydrated lirne being the most preferred.

The quantity of bonding agent used in the admixture may range from about1% up to as high as or more, based on the total weight of the dry solidsin the admixture. Concentrations of bonding agents lower than about 1%produces a hardened agglomerate which has inadequate strength forhandling. The upper limit of the concentration of the bonding agent isdictated primarily by the quantity of silica and other slag formingimpurities in the collected BOF dust, and the other steel mill wastefines when used. Amounts in excess of this level do not appreciablyimprove the strength of the hardened agglomerates and dilutes theconcentration of iron therein. Preferably, the concentration of bondingagent in the admixture is in the range of about 1 to about 15, mostpreferably about 3 to about 7, weight percent, based on the total weightof the dry solids in the admixture.

In addition to the steel mill dust, the bonding agent, and thecarbonaceous material, other strengthening additives can be added to theadmixture to increase the strength of the resultant hardenedagglomerate. For example, oxides, hydroxides, carbonate, bicarbonatessulfites, bisulfateand borates of the alkali metals (e.g. potassium andsodium) and mixtures thereof can be added in amounts ranging up to about1%, calculated as the oxide. Of these, sodium hydroxide, sodiumcarbonate and sodium bicarbonate are preferred. Al-.

though the concentrations of these materials in excess of 1% willprovide further strengthening, such higher concentrations may possiblyproduce corrosion to the linings of most steel making furnaces duringsubsequent smelting of the agglomerates. Concentrations as low as 0.10weight percent, based on the total weight of the dry solids in theadmixture, produce a measurable increase in agglomerate strength;howeve'r,'it is generally preferred to include about 0.15 to about 1 Iweight percent of these strengthening agents.

These strengthening agents are preferably added to the admixture in theform of an aqueous solution which may range in strength andconcentration from about 10 to about 75% by weight and preferably a 50%solution. The strengthening agent also can be added in dry form asfinely divided particles, but due to their hygroscopicity, toxicity andcorrosivity, it is preferred to employ aqueous solutions which providethe'further advantage of easier handling and uniform distributionthroughout the admixture. The particular concentration of the aqueoussolution of the strengthening agents can be varied consistent withconsideration of such factors as the water content in the initial greenmixture and the optimum water content desired for the specific type ofmolding, pelletizing, briquetting or extruding operation employed forforming the green agglomerates.

Natural or artificial siliceous materials containing silica, which willreact with the fluxing agent under the hydrothermal conditions of thehardening step to form silicate bonds therewith, can be used in lieu of,or in addition to, the above strengthening agents in concentrationsranging from about 0.1 to about 10, preferably about 0.5 to about 3,weight percent, based on the total dry weight of the solids in theadmixture. Representative examples of acceptable siliceous materialsinclude bentonite, diatomaceous earth, fullers earth, portland cement,sodium silicate, pyrogenic silicas, hydrated silicas, ground quartz,silica sand, and calcium, magnesium, and aluminum silicates. Of these,ground quartz and silica sand are preferred.

The average size and distribution of the carbonaceous material, bondingagent and other solid additives included in the admixture, in additionto the water content of the admixture, will vary depending on thecomposition and size of the dust particles and the particular molding,pelletizing, briquetting'or extruding technique employed to form theadmiture into green agglomerates. For exmple, when a pelletizing processusing a drum or disc to form spherical pellets of the desired size isused, it is generally preferred to control the water content of theadmixture within the range of about 3 to about 10 weight percent, basedon the total weight of the moistened admixture. On the other hand, whenthe briquetting roll press is used, moisture contents of about 4 toabout 6 weight percent are preferred and the size of the particle andsize distribution is less important. It can be appreciated that theparticle size range in the mixture as well as the quantity of watercontaind therein can be optimized for each specific type of dust andtechnique employed for forming the green agglomerate. The particle sizeof the carbonaceous material, the bonding agent and the other solidadditives can generally range from about 60 mesh to about 400 mesh,preferably all less than about 270 mesh. Particle sizes coarser thanabout 20 mesh increases the difficulty of obtaining a homogeneous mitureof the constituents and, in some cases, provides insufficient surfacearea to obtain the requisite high strength bond in the resultanthardened agglomerate. Hence, the size of the materials is preferablycontrolled within the ranges below about 60 mesh and preferably with atleast half of the material in sizes less than 270 mesh.

After the admixture has been thoroughly mixed to homogeneously dispersethe constituents, it is subjected to a molding, briquetting, pelletingor extruding operation to form green agglomerates of the desired sizeand configuration providing optimum strength and smeltingcharacteristics. The agglomerates are preferably formed into relativelycompact configurations, such as cylinders, spheres, egg shapes, pillows,etc., and substantially devoid any thin sections or sharp angularitieswhich might be susceptible to fracture or breakage during handling. Theconfiguration of the agglomerates is also controlled so that they do notbecome tightly compacted when stacked together and thereby prevent thepassage of a heated moisture-laden fluid therethrough during thesubsequent hydrothermal hardening step. Preferably, the agglomeratesarein the form of spheres having a diameter ranging from about 5 toabout 40, most preferably about 10 to about 15, millimeters.

The green agglomerates usually lose a portion of their moisture contentduring handling and forming.

Moisture content less than about 3 weight percent generally do notprovide sufficient cohesion of the mass, whereas moisture content inexcess of about 12 weight percent are sometimes undesirable because themass is too plastic to retain a formed shape. Hence, moisture content ofthe green agglomerates prior to the forming step is preferablycontrolled within the range of about 3 to about weight percent and, morepreferably, from about 6 to about 8 weight percent.

A problem often encountered in the hydrothermal hardening of BOFagglomerates is that the BOP dust (and other steel mill waste fines whenused) frequently contains appreciable amounts of overburnt lime or slagparticles. These materials will cause swelling and degradation of theagglomerates during autoclaving. To overcome this problem the mixture tobe used as feed for agglomeration should be aged for a sufficient timefor all of this overburnt lime and/or slag to become hydrated before themixture is formed into green agglomerates. For example, this hydrationcan be advantageously accomplished by heating the feed mixture within aclosed container, in the presence of a moist atmosphere, to about 80C toabout 90C forabout 12 to 24 hours. Lower heating temperatures andshorter times can be used; however, a lesser degree of hydration occurswith a resultant lower strength of the agglomerate. If desired thisaging can be performed in an autoclave at higher temperatures andpressure for a shorter period of time. However, use of this technique isgenerally less preferred because of the higher operational costsrequired. It can be appreciated that the temperature and time requiredto obtain the desired hydration is largely dependent upon theconcentration of overburnt lime and/or slag in the BOP dust and othersteel mill waste fines used and the particle size of these materials,with longer times being required for higher concentrations and largerparticles.

After being sized to a predetermined, substantially uniform sizeadvantageous for charging to a steel making furnace, the formed greenagglomerates can be charged directly into a chamber or pressure vessel,such as an autoclave, wherein they are heated to an elevated temperatureunder pressure in the presence of moisture to effect a hardening andbonding of individual particles into an integral, high strength agglomenhardening step. This drying can be accomplished in a conventional dryingapparatus, such as an oven or open flame device, in the presence ofambient air with the drying temperature used being substantially belowthe combustion or decomposition temperature of the carbonaceousmaterial. The strength of the resultant agglomerate depends to someextent upon the pressure, temperature, time and moisture content duringthe hydrothermal hardening step. By increasing the pressure and/ortemperature, stronger bonds can be obtained for the same length of timeor agglomerates of equal strength can be produced in a shorter time. Itcan be appreciated that the time, pressure, temperature and amount ofbonding agent and/or other strengthening materials required to producean agglomerate from a specific type of steel mill dust are interrelatedand these variables must be optimized in accordance with availableequipment and economic considerations to provide a hardened agglomerateof the desired physical and chemical characteristics.

The application of heat to the green or unhardened agglomerates may beachieved in any one of a number of methods. The use of steam ispreferred because it simultaneously provides a source of heat andmoisture necessary for the hydrothermal reaction. Either saturated steamor superheated steam can be used. However, use of superheated steam athigh temperatures produces a dry condition which results in weakeragglomerates. Therefore, it is preferable to use superheated steam withtemperatures and pressures close to that of saturated steam.

Temperatures generally ranging from about 200F to about 700F, dependingupon the particular carbonaceous material used, can be satisfactorilyemployed to achieve a hardening of the green agglomerates withinreasonable time periods. The maximum temperature used should be belowthat at which fusion or unwanted thermal decomposition of thecarbonaceous material and other constituents occur. When thecarbonaceous material used contains substances which decompose atrelatively low temperatures (e.g. bituminous coal), the maximumtemperature should not exceed about 550F. When coke is used as thecarbonaceous material, tempe'ratures as high as 700F and even higher canbe used without effecting an undesirable thermal decomposition. Thelowest temperature suitable for initiating a hydrothermal reaction isabout 200F. In view of economic considerations relating to the curingtime required for achieving an acceptable bond strength, a lowertemperature of about 300F, preferably about 400F, is used.

Although the hydrothermal reaction can be achieved at atmosphericpressure, it is preferred to employ pressures greater than atmospheric,i.e., up to pressure approaching the capacity of a conventional pressurevessel, in order to decrease the curing time and to improve the strengthof the resultant cured agglomerate.

The use of this hydrothermal hardening step is particularly advantageousbecause the low hardening temperature (and thus the lower operatingcosts) required and the neutral atmosphere thereof insures that thezinc, lead and carbonaceous material is retained in the agglomerates.Hence, gaseous pollutants are not emitted and the hardened agglomeratesintegrally contain sufficient solid reductant to effect the desiredreduction in the subsequent heating step. Furthermore, the hardenedagglomerates have a sufficient compressive strength (e.g. generallyranging from about to about 600 lbs.) to permit them to be handled quiteextensively and even transported considerable distances to off-sitesteel making processes, if desired.

The compressive strength of the hardened agglomerate can be increased byrapidly drying them immediately after removal from the hydrothermalhardening apparatus instead of allowing them to cool and dry at ambientconditions. This drying can be accomplished at a temperature in therange of about F to about 225F for 5 to 60 minutes, with the preferredtemperature and time being about 200F and 15 minutes, respectively.

' In some basic oxygen steel making processes where the zinc and leadcontent in the agglomerates will not cause the total content of theseimpurities in the fur nace to exceed tolerable levels, the hardenedagglomerates can be recycled to the process without further treatment.In this case, the iron in the agglomerates is metallized and recoveredwith the remainder of the iron. The zinc and lead in the agglomeratesvolatilize and again report in the BOP dust. After a number of suchcycles, the zinc and lead content in the furnace will eventually exceedtolerable levels. For example, if the normal amount of zinc in the BOPdust is 0.5 weight percent, its content will build up to about 13 weightpercent after 26 cycles. it has been shown that some BOF furnaces can beoperated with the dust containing zinc up to about 13 weight percentwithout any deleterious effects. After this level has been reached, aportion of the dust containing an amount of zinc corresponding to thatadded to the process by the scrap metal charge can be routed to aminiaturized zinc re-v covery plant to remove zinc, lead, and otherreducible impurities therefrom. The thus-treated agglomerates can berecycled, along with the untreated, hardened agglomerates, back to theBOP furnaces. in this manner, the zinc and lead content in the BOP neverbuilds up to an intolerable level.

The zinc recovery plant used can consist of a conventional zincretorting apparatus, such as a shaft furnace. In the retort step theagglomerates are heated to a temperature in the range of about 1,9001Fto about 2,400F, preferably about 2,000F to about 2,300F, for asufficient time to reduce the iron oxide to metallic iron and to reducethe volatilize zinc, lead and other reducible impurities. Since theagglomerates contain sufficient free carbon for this reductioninternally, the kinetics are such that a very rapid reduction can beeffected. Hence, the total heat energy required is sub stantiallyreduced with a corresponding reduction in overall operational costs. Theheating time required to effect this reduction depends primarily uponthe amount iron oxide, zinc, lead, fluxing agents and carbonaceousmaterial contained in the agglomerates and the particular retorttemperature used. As a guide, with a retort temperature of about 2,300F,the heating time required to effect a substantially completemetallization and volatilization of zinc and lead will be in the rangeof about-.15 to about 30 minutes.

When it is desired to use the agglomerates as a direct charge to a blastfurnace or an electric steel making furnace where the presence ofappreciable amounts of zinc and/or lead is intolerable, the hardenedagglomerates must be first retorted in the general manner describedabove. If recovery of zinc is not considered feasible at the steelmaking plant facility, the untreated, zinc-rich, hardened agglomeratescan be transported to a zinc smelting facility for retorting.

The following examples are presented to illustrate the process of thisinvention and are not to be construed as limitations thereto.

EXAMPLE 1 Material Weight BOF dust" 87 Ca(OH), (-325 mesh) SiO, (-325mesh) 3 Carbon (-325 mesh) 4.33 Water 1O BOF dust analysis: 5.1 weightpercent Zn, 57.1 weight percent Fe "Carbon added as anthracite coalfines containing 75.3% fixed carbon Green, spherically-shaped pellets,approximately 15 mm in diameter and having a moisture content of 10.4%,were formed from this mixture with a conventional balling device. Thepellets were then hardened in a 5-liter Cenco-Megnel autoclave for 1hour at 300 psig in a saturated steam atmosphere. The hardened pelletshad an average compressive strength of about 228 lb.

Zirconium boats containing seven hardened pellets each were placed in aLeco 2600 tube furnace and the pellets were heated therein at varioustemperatures and for various time periods. After cooling the reducedpellets were analyzed to determine the degree of zinc removal. Theresults of these tests were as follows:

% Zinc Removal Reduction Temp., F. Reduction Time,

min. 1900 5 7 1900 30 2l 2100 5 18 2100 30 46 2300 5 36 2300 30 Fromthese test results, it can be see that, even though a substantial amountof zinc was removed from pellets prepared from a mixture containing lessthan 5% carbon within a reasonably short time period, considerable zincremained even though the pellets were heated to 2,300F for 30 minutes.

EXAMPLE 2 A second batch of hardened pellets was prepared insubstantially the same manner as in Example 1, except the BOlF dustcontent in the moistened mixture was 77% and the carbon content was 11.2%. The hardened pellets had an average compressive strength of 207lb.

The hardened pellets were reduced in the same manner described inExample 1. After cooling the reduced pellets were analyzed to determinedegree of zinc removal and the degree of iron metallization wasdetermined for some of the pellets. The results of these tests were asfollows:

Reduction Reduction Zinc lron Temp, "F. Time, min. Removed Metal- 1lization 1900 5 19 n.a. 1900 15 72. n.a. 2 5 7 l 76 2 l 00 15 98 94 23005 96 86 2300 i5 100 l 00 From these results it can be seen that, byusing 1 1.2 weight percent carbonaceous material in the startingmixture, sufficient carbon remained in the high strength, hardenedpellets to permit substantially all the zinc to be removed within 15minutes heating time at a temperature of 2,1002,300F. Also, the pelletswere substantially completely'metallized at these same conditions.Furthermore, these results show that substantial amounts of zinc removaland metallization can be 0 effected at temperatures and heating timeslower than these more preferred levels.

Tests have been run where mill scale and other steel mill waste dustsare combined with BOF dust with similar results relative to zinc removaland metallization.

Because of the high carbon contents of some steel mill in the properproportions to obtain an equivalent fixed carbon content in theresultant mixture within the range of about to 25 weight percent. Inthis case it is not necessary to add an additional carbonaceous materialto the mixture.

From the above detailed description, it can be appreciated that thisinvention provides an inexpensive method for forming zinc-rich steelmill waste dusts into crush-resistant, hardened agglomerates which canbe easily and rapidly reduced to metallize them and to remove zinc, leadand other reducible impurities.

I claim:

l. A method for making hardened agglomerates suitable for charging to asteel-making furnace from steel mill waste dusts comprising the stepsof:

a. forming a moistened mixture including a finelydivided waste dustrecovered from the fumes of a steel-making furnace and containing iron,zinc and overburnt lime, a finely divided solid carbonaceous materialand a bonding agent selected from the group consisting of the oxides,hydroxides and carbonates of calcium andmagnesium and mixtures thereof,the moisture content of said mixture being in the range of about 3 toabout 15 weight percent, said carbonaceous material containingsufficient carbon to make the total equivalent fixed carbon of saidmixture in the range of about 5 to about 25 weight percent, based on thetotal weight of the dry solids in said mixture, and the amount of saidbonding agent being in the range of about 1 to, about 25 weight percent,based on the total weight of the dry solids in said mixture,

b. aging the resultant mixture at an elevated temperature for asufficient time to hydrate substantially all of the overburnt limetherein; 7

c. forming the aged mixture into discrete agglomerate masses;

d. hydrothermally hardening said agglomerates under steam pressure at atemperature below the combustion and decomposition temperature of saidcarbonaceous material for a time period sufficient to form hard andintegrally bonded masses, said hardened agglomerates containingsubstantially all of the free carbon introduced into said mixture bysaid carbonaceous material.

2. The method according to claim 1 further comprising the step of:

e. heating said hardened agglomerates to a temperature within the rangeof about 1,900F to about 12 2,400F for a sufficient time to removesubstantially all of the zinc therein.

3. The method according to claim 1 wherein said waste dust is recoveredfrom the fumes from a basic oxygen furnace.

4. The method according to claim 3 wherein said mixture includes atleast one other finely divided iron containing waste material selectedfrom the group consisting of open hearth dust, scarfer grit, mill scale,and spark box fines.

5. The method according to claim 1 wherein said bonding agent is calciumoxide, calcium hydroxide, or a mixture thereof.

6. The method according to claim 5 wherein said mixture includes asiliceous material selected from the group consisting of bentonite,diatomaceous earth, fullers earth, portland cement, sodium silicate,pyrogenic silicas, hydrated silicas, calcium silicate, magnesiumsilicate, aluminum silicate, ground quartz, silica sand and mixturesthereof in an amount within the range of about 0.1 to about 10 weightpercent, based on the total weight of the dry solids in said mixture.

7. The method according to claim 6 wherein step (b) is carried out in amoist atmosphere at a temperature of about 80F to about 90C for about 12to about 24 hours.

8. The method according to claim 6 wherein step (d) is carried out inthe presence of a saturated steam atmosphere at a temperature in therange of about 200F to about 550F.

9. The method according to claim 8 including the 7 step of drying saidagglomerates to reduce the moisture content thereof to a maximum ofabout 3 weight percent prior to step (d).

10. The method according to claim 9 including the step of drying saidhardened agglomerates immediately after step (d) at a temperature ofabout F to about 250F for about 5 to about 60 minutes.

11. The method according to claim 5 wherein said mixture includes silicain an amount within the range of about 0.1 to about 3 weight percent,said bonding agent in an amount within the range of about 1 to about 15weight percent, and said carbonaceous material in an amount within therange of about 10 to about 15 weight percent, all based on the totalweight of the dry solids in said mixture.

2. The method according to claim 1 further comprising the step of: e.heating said hardened agglomerates to a temperature within the range ofabout 1,900*F to about 2,400*F for a sufficient time to removesubstantially all of the zinc therein.
 3. The method according to claim1 wherein said waste dust is recovered from the fumes from a basicoxygen furnace.
 4. The method according to claim 3 wherein said mixtureincludes at least one other finely divided iron containing wastematerial selected from the group consisting of open hearth dust, scarfergrit, mill scale, and spark box fines.
 5. The method according to claim1 wherein said bonding agent is calcium oxide, calcium hydroxide, or amixture thereof.
 6. The method according to claim 5 wherein said mixtureincludes a siliceous material seleCted from the group consisting ofbentonite, diatomaceous earth, fuller''s earth, portland cement, sodiumsilicate, pyrogenic silicas, hydrated silicas, calcium silicate,magnesium silicate, aluminum silicate, ground quartz, silica sand andmixtures thereof in an amount within the range of about 0.1 to about 10weight percent, based on the total weight of the dry solids in saidmixture.
 7. The method according to claim 6 wherein step (b) is carriedout in a moist atmosphere at a temperature of about 80*F to about 90*Cfor about 12 to about 24 hours.
 8. The method according to claim 6wherein step (d) is carried out in the presence of a saturated steamatmosphere at a temperature in the range of about 200*F to about 550*F.9. The method according to claim 8 including the step of drying saidagglomerates to reduce the moisture content thereof to a maximum ofabout 3 weight percent prior to step (d).
 10. The method according toclaim 9 including the step of drying said hardened agglomeratesimmediately after step (d) at a temperature of about 175*F to about250*F for about 5 to about 60 minutes.
 11. The method according to claim5 wherein said mixture includes silica in an amount within the range ofabout 0.1 to about 3 weight percent, said bonding agent in an amountwithin the range of about 1 to about 15 weight percent, and saidcarbonaceous material in an amount within the range of about 10 to about15 weight percent, all based on the total weight of the dry solids insaid mixture.